1. Field of the Invention
The present invention relates to a transmission power control circuit, more particularly, a transmission power control circuit which controls transmission power of a transmitter used in a mobile communication system such as an automobile telephone system, a portable telephone system, and the like.
2. Description of the Related Art
In a mobile communication system, transmission power of the mobile terminal is controlled in response to electrical field strength which varies with the distance between the mobile terminal and a base station and other conditions, to prevent the mobile terminal from transmitting needlessly, and to reduce power consumption of the mobile terminal.
In order to perform this function, a radio transmitter for the mobile terminal is provided with an automatic power control circuit (APC). The APC controls the amount of amplification of a power amplifier (PA) so that a monitor voltage corresponding to the transmission power of the PA becomes equal to a reference voltage corresponding to a required transmission power, and thus, the transmission power is controlled to be a required value. As the reference voltage can be discretely altered according to a digital control signal, the transmission power is controlled stepwise.
An AM detector for detecting the level of transmission power is provided in the PA, in order to generate the monitor voltage signal. If there is a certain deviation in the characteristics of elements used for the AM detector from standard characteristics, a different monitor voltage can be generated, and thus the level of the transmission power becomes incorrect. Therefore, the transmission power of the products is adjusted before shipping. The adjustment of the transmission power is carried out by altering values of reference voltages V.sub.1 and V.sub.2, which correspond to the minimum and the maximum transmission powers, respectively, with two variable resistors.
In the aforementioned conventional transmission power control circuit, another problem to be solved arises in the mobile communication system where a carrier frequency of the transmission wave is frequently changed in accordance with an assigned frequency channel. In this case, though the values of the reference voltages V.sub.1 and V.sub.2 are correctly adjusted for one frequency channel, the transmission power can be changed for another frequency, because there is a frequency dependency in the characteristics of the elements forming the AM detector.
Additionally, there can be dispersion in the frequency dependency of the elements. Therefore, it is also desirable to overcome this problem.
Moreover, in the aforementioned conventional transmission power control circuit, the level of the transmission power can be incorrect in a medium range even if the maximum and the minimum transmission power are correctly adjusted.